Stock-Based Compensation - Stock Plans (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Stock Plans [Abstract]
Shares of common stock added	93,885,414	93,885,414		89,345,978
Unrecognized compensation expense related to non-vested stock options	$ 72.8	$ 72.8
RSUs [Member]
Stock Plans [Abstract]
Unrecognized compensation expense related to RSUs	$ 14.8	$ 14.8
Weighted average amortization period		2 years 6 months
Non-vested Stock Options [Member]
Stock Plans [Abstract]
Weighted average amortization period		1 year 4 months 24 days
2015 Equity Incentive Plan [Member]
Stock Plans [Abstract]
Number of shares authorized for issuance pursuant to stock awards (in shares)	18,500,000	18,500,000
Number of shares available for future grant (in shares)	3,596,826	3,596,826
Amount of reversal of stock-based compensation previously recognized	$ 18.8
2015 Equity Incentive Plan [Member] | Stock Options [Member]
Stock Plans [Abstract]
Options outstanding (in shares)	10,135,657	10,135,657
Options exercisable (in shares)	3,335,196	3,335,196
2015 Equity Incentive Plan [Member] | RSUs [Member]
Stock Plans [Abstract]
Restricted stock units outstanding (in shares)	616,643	616,643
2017 Employee Stock Purchase Plan [Member]
Stock Plans [Abstract]
Number of shares available for future grant (in shares)	1,428,725	1,428,725
Shares of common stock added			500,000
Purchase price of common stock, percent		85.00%
Number of shares of common stock reserved for issuance (in shares)	1,500,000	1,500,000
Shares issued under ESPP (in shares)	21,984	39,724
Accrued liability for contributions to employee stock purchase plan	$ 0.2	$ 0.2